In the manufacture of absorbent paper products such as facial tissue, bath tissue, paper towels, napkins and the like, many different sheet properties influence the performance of the particular product being made. Softness, strength, absorbency, bulk and the like are often the subject of improvements. However, a property of tissue-related products is that when wetted and crumpled in the hand, they essentially collapse into a dense wet mass. Stated another way, such tissue products have a low wet compressive modulus, low bending modulus and low wet resiliency. These properties are undesirable for such products when used to wipe up liquids because, once saturated, they lose their designed structure and thus much of their functionality.
Similar problems can be found with some disposable absorbent articles. Generally, disposable absorbent articles include, in their construction, an absorbent core positioned between a liquid-permeable cover or topsheet layer and a liquid-impermeable baffle or backsheet layer. The cover material is generally designed to allow body exudates to permeate through the cover so that the absorbent core can absorb the fluids. The baffle or backsheet material is generally fluid impermeable and is positioned so that it is away from the body. The absorbent core serves to store fluid that contacts the article. An additional layer of material, termed a transfer layer or surge layer, may also be present between the absorbent core and the liquid-permeable cover. This layer serves to manage the transfer or distribution of the liquid to the absorbent core. Examples of such absorbent articles include products such as diapers, sanitary napkins, training pants, incontinent garments, overnight pads, panty liners, underarm shields, as well as other absorbent devices used for medical purposes such as surgical absorbents. Such articles are designed to absorb body fluids, such as urine, menses, blood, perspiration and other excrement discharged by the body.
One continuing problem with some disposable absorbent articles is that the bodily excretions are usually directed at one portion of the absorbent, whereas the absorptive capacity of the product is spread over a greater area. Localized insults of body fluid may cause a failure of the product because the fluid handling characteristics of the liquid-permeable cover, transfer layer and the absorbent core are inadequate to quickly distribute the fluid throughout the absorbent core material. Such failures are often in part due to the collapse of the low density structure of the various components when wetted. This is a particular problem for cellulosic materials.
Accordingly, there is a need for a wet-resilient web material that can more effectively transfer and/or absorb fluids for use in tissues, towels and absorbent products.
It has been discovered that papermaking fibers containing high-yield fibers, such as chemithermomechanical pulp fibers, when combined with wet strength additives, can be made into a low-density, three-dimensional sheet or web followed by or incorporating largely noncompressive drying means such that the resulting low density cellulosic sheet has remarkable wet resiliency properties, showing great resistance to wet collapse.
xe2x80x9cNoncompressive dryingxe2x80x9d refers to drying methods such as through-air drying; air jet impingement drying; non-contacting drying such as air flotation drying, as taught by E. V. Bowden, Appita Journal, 44(1): 41 (1991); through-flow or impingement of superheated steam; microwave drying and other radiofrequency or dielectric drying methods; water extraction by supercritical fluids; water extraction by nonaqueous, low surface tension fluids; infrared drying; drying by contact with a film of molten metal; and other methods for drying cellulosic webs that do not involve compressive nips or other steps causing significant densification or compression of a portion of the web during the drying process. (Standard dry creping technology is viewed as a compressive drying method since the web must be mechanically pressed onto part of the drying surface, causing significant densification of the regions pressed onto the heated Yankee cylinder.) The three-dimensional sheets of the present invention could be dried with any of the above mentioned noncompressive drying means without causing significant web densification or a significant loss of their three-dimensional structure and their wet resiliency properties.
Preferably, the low-density three-dimensional structure is created in substantial part before the sheet reaches a solids level (dryness level) of about 80% or higher. Creating the low-density three-dimensional structure can be achieved in part through a variety of means, including but not limited to the use of specially treated high-bulk fibers such as curled or chemically treated fibers as an additive in the furnish, including the fibers taught by C. C. Van Haaften in xe2x80x9cSanitary Napkin with Cross-linked Cellulosic Layer,xe2x80x9d U.S. Pat. No. 3,339,550, issued Sep. 5, 1967, which is hereby incorporated by reference; mechanical debonding means such as differential velocity (xe2x80x9crushxe2x80x9d) transfer between fabrics or wires, hereafter described; mechanical straining or xe2x80x9cwet strainingxe2x80x9d of the moist web, including the methods taught by M. A. Hermans et al. in U.S. Pat. No. 5,492,598, xe2x80x9cMethod for Increasing the Internal Bulk of Throughdried Tissue,xe2x80x9d issued Feb. 20, 1996, herein incorporated by reference, and M. A. Hermans et al. in U.S. Pat. No. 5,411,636, xe2x80x9cMethod for Increasing the Internal Bulk of Wet-Pressed Tissue,xe2x80x9d issued May 2, 1995, herein incorporated by reference; molding of the fiber onto a three-dimensional wire or fabric, such as the fabrics disclosed by Chiu et al. in U.S. Pat. No. 5,429,686, xe2x80x9cApparatus for Making Soft Tissue Products,xe2x80x9d issued Jul. 4, 1995, which is hereby incorporated by reference, including differential velocity transfer onto or from said three-dimensional wire or fabric; wet embossing of the sheet; wet creping; and the optional use of chemical debonding agents.
Products of the present invention have surprisingly high wet resiliency. For example, when the products of this invention are saturated with water and crumpled in one""s hand into a ball about the size of a golf ball, and thereafter released, they quickly open up to mostly uncrumple themselves. By contrast, current commercially-available products such as bath tissues and paper towels remain substantially wadded up in a wet ball. It has been further discovered that such sheets, when properly made, can have unexpectedly good fluid handling properties, such as high intake rate, high in-plane permeability, high absorption capacity, and rapid in-plane distribution of liquid, making these materials ideally suited for use in tissues, paper towels and numerous absorbent articles. As used herein, unless otherwise stated, absorbent articles include sanitary napkins and other feminine care products; disposable diapers and related personal care products; training pants; incontinence products; breast pads; poultry pads and meat pads for absorbing blood and meat juices; bed pads for home and hospital use; sweat bands and other perspiration absorbing articles; odor and sweat absorbing pads for use in shoes or garments; and the like. The materials of the present invention can be utilized in numerous articles where fluid is absorbed or entrapped, functioning as fluid surge webs, transfer layers, distribution webs, absorbent cores, absorbent composites, and so forth. The high wet strength and significant large-scale texture of the materials also can serve effectively in preventing the breakup or loss of integrity of weaker, adjacent materials such as fluff pulp or tissue in absorbent articles, allowing the materials of the present invention to serve effectively as means for maintaining or improving the integrity of the absorbent core (superabsorbent/fluff mixture) of absorbent articles such as diapers and the like. Further, the combination of high wet strength, high absorption capacity, and significant surface texture makes these materials ideally suited for cleaning operations such as scrubbing, mopping, and wiping, with possible incorporation into cleaning articles such as mops, wipers, scrub pads, and the like. As used herein, the terms xe2x80x9cwebxe2x80x9d and xe2x80x9csheetxe2x80x9d are used interchangeably and mean the same.
The unique properties and characteristics of the sheets of this invention can be quantified by one or more of the following terms, which will hereinafter be described and defined: Overall Surface Depth; wet:dry ratio; Wet Wrinkle Recovery Test; Wet Compressed Bulk; Wet Springback ratio; Loading Energy Ratio; Compression Ratio: In-Plane Permeability; the FIFE Test; Dry Wipe Residual Total Area and Mass Factor; Wet Wipe Residual Total Area and Mass Factor; Mean Volume-Weighted Pore Length; and the Thickness Variation Index. All of these terms relate to the superior performance of the sheets of this invention when used in various product applications.
Hence, in one aspect, the invention resides in a non-compressively dried cellulosic web, such as a through-air-dried web, more specifically an uncreped through-air-dried web, having a density of about 0.3 gram per cubic centimeter or less and a three-dimensional surface having an Overall Surface Depth of about 0.10 millimeter or greater, said web comprising a wet strength agent and at least about 10 dry weight percent high yield pulp fibers, preferably virgin high yield pulp fibers, and more preferably virgin softwood fibers, said three-dimensional surface preferably being created in substantial part by mechanical means prior to reaching a dryness level of about 80 percent, more preferably with the use of through-drying fabrics, and preferably with a rush transfer level exceeding 10 percent.
The basis weight of the webs of this invention can be about 10 grams per square meter (gsm) or greater, more specifically from about 10 to about 80 gsm, still more specifically from about 20 to about 60 gsm, and still more specifically from about 30 to about 50 gsm.
The fiber composition of the webs of this invention can have from about 10 to about 100 percent wood pulp fibers, particularly containing about 70 percent or greater, more specifically about 80 percent or greater, more specifically about 90 percent or greater, and still more specifically about 95 percent wood pulp fibers or greater. Additionally, it is preferred that the fiber composition of the webs of this invention comprise about 70 percent or greater softwood fibers, more specifically about 80 percent or greater, and still more specifically about 90 percent or greater softwood fibers.
In another aspect, the present invention resides in an absorbent article comprising a backsheet layer, a liquid permeable topsheet layer connected in a superposed relation with the backsheet layer, and at least one cellulosic web as described above sandwiched between the topsheet layer and the backsheet layer. The cellulosic web can also serve as an absorbent core material to retain and store liquid, particularly when incorporated into the absorbent article in multiple plies (such as from about 2 to about 20 or more, more specifically from about 2 to about 5 or 10) or it can serve to receive and distribute liquid to the absorbent core by being positioned in liquid communication with the absorbent. As such the webs of this invention can be used as xe2x80x9ctransferxe2x80x9d layers, xe2x80x9csurgexe2x80x9d layers, xe2x80x9cdistributionxe2x80x9d layers and the like.
Representative patents illustrating absorbent products in which the web or sheets of this invention can be used include: U.S. Pat. No. 5,386,595 issued Feb. 7, 1995 to Kuen et al. entitled xe2x80x9cGarment Attachment Systemxe2x80x9d; U.S. Pat. No. 4,500,316 issued Feb. 19, 1985 to Damico entitled xe2x80x9cDisposable Garmentxe2x80x9d; U.S. Pat. No. 5,364,382 issued Nov. 15, 1994 to Latimer et al. entitled xe2x80x9cAbsorbent Structure Having Improved Fluid Surge Management and Product Incorporating Samexe2x80x9d; U.S. Pat. No. 4,940,464 issued Jul. 10, 1990 to Van Gompel et al. entitled xe2x80x9cDisposable Incontinence Garment or Training Pantxe2x80x9d; and copending application Ser. No. 415,382 filed Apr. 3, 1995 in the names of D. Fries et al. entitled xe2x80x9cAbsorbent Article With Laminated Tapexe2x80x9d, all of which are herein incorporated by reference.
Although the reasons for the unexpectedly good material properties and product performance results obtained with the present invention are not fully understood, it appears that three factors interact synergistically to yield unusually high wet resiliency performance: (1) a high bulk (low density) three-dimensional structure obtained without significant compression during drying and preferably obtained without creping, (2) high yield pulp fibers, preferably comprising at least about 20 percent of the fiber furnish used to make the sheet; and (3) the use of one or more wet strength resins or agents such that the wet to dry geometric mean tensile strength ratio is about 0.1 or greater. It has been found that if any of these three factors is missing, a wetted sheet will lack the high wet resiliency and/or other properties which are important for many of the uses for the webs of the present invention.
In describing the webs of this invention and their fluid-handling characteristics, a number of terms and tests are used which are described below.
As used herein, xe2x80x9chigh yield pulp fibersxe2x80x9d are those natural papermaking fibers produced by pulping processes providing a yield of about 65 percent or greater, more specifically about 75 percent or greater, and still more specifically from about 75 to about 95 percent. Yield is the resulting amount of processed fiber expressed as a percentage of the initial raw material mass. Such pulping processes include bleached chemithermomechanical pulp (BCTMP), chemithermomechanical pulp (CTMP) pressure/pressure thermomechanical pulp (PTMP), thermomechanical pulp (TMP), thermomechanical chemical pulp (TMCP), high yield sulfite pulps, and high yield kraft pulps, all of which leave the resulting fibers with high levels of lignin. High yield fibers are well known for their stiffness (in both dry and wet states) relative to typical chemically pulped fibers. The cell wall of kraft and other non-high yield fibers tends to be more flexible because lignin, the xe2x80x9cmortarxe2x80x9d or xe2x80x9cgluexe2x80x9d on and in part of the cell wall, has been largely removed. The preferred high yield pulp fibers can also be characterized by being comprised of comparatively whole, relatively undamaged fibers, high freeness (250 Canadian Standard Freeness (CFS) or greater, more specifically 350 CFS or greater, and still more specifically 400 CFS or greater), and low fines content (less than 25 percent, more specifically less than 20 percent, still more specifically less that 15 percent, and still more specifically less than 10 percent by the Britt jar test). Webs made with recycled fibers are less likely to achieve the wet resiliency properties of the present invention because of damage to the fibers during mechanical processing. In addition to common papermaking fibers listed above, high yield pulp fibers also include other natural fibers such as milkweed seed floss fibers, abaca, hemp, cotton and the like. Fibers from wood are preferred.
The amount of high yield pulp fibers in the sheet can be at least about 10 dry weight percent or greater, more specifically about 15 dry weight percent or greater, more specifically about 30 dry weight percent or greater, still more specifically about 50 dry weight percent or greater, and still more specifically from about 20 to 100 percent. For layered sheets, these same amounts can be applied to one or more of the individual layers such that the overall unitary web has at least about 10 or 15 percent high yield fibers. Because high yield pulp fibers are generally less soft than other papermaking fibers, in some applications it is advantageous to incorporate them into the middle of the final product, such as placing them in the center layer of a three-layered sheet or, in the case of a two-ply product, placing them in the inwardly-facing layers of each of the two plies.
xe2x80x9cWater retention valuexe2x80x9d (WRV) is a measure that can be used to characterize some fibers useful for purposes of this invention. WRV is measured by dispersing 0.5 grams of fibers in deionized water, soaking overnight, then centrifuging the fibers in a 1.9 inch diameter tube with a 100 mesh screen at the bottom at 1000 G for 20 minutes. The samples are weighed, then dried at 105xc2x0 C. for two hours and then weighed again. WRV is (wet weightxe2x80x94dry weight)/dry weight. Fibers useful for purposes of this invention can have a WRV of about 0.7 or greater, more specifically about 0.9 or greater, still more specifically from about 0.9 to about 2. High yield pulp fibers often have a WRV of about 1 or greater.
xe2x80x9cDensityxe2x80x9d can be determined by measuring the caliper of a single sheet using a TMI tester with a load of 0.289 psi. Density is calculated by dividing the caliper by the basis weight of the sheet. The webs of this invention commonly have low, substantially uniform densities (high bulks). Substantial density uniformity can be achieved, for example, by noncompressive drying means such as throughdrying to final dryness without differentially compressing the web. While the webs of this invention have a three-dimensional contour imparted by the topography of a throughdrying fabric, the side-to-side thickness of the web is relatively uniform. In general, the density of the products of this invention can be about 0.3 gram per cubic centimeter or less, more specifically about 0.15 gram or less, still more specifically about 0.1 gram per cubic centimeter or less. It is believed to be important that the absorbent structure, once formed, be dried without substantially reducing the number of wet-resilient interfiber bonds. Throughdrying, which is a common method for drying tissues and towels, is a preferred method of preserving the structure. Absorbent structures made by wet laying followed by throughdrying typically have a density of about 0.1 gram per cubic centimeter, whereas airlaid structures normally used for diaper fluff typically have densities of about 0.05 gram per cubic centimeter. All of such structures are within the scope of this invention.
xe2x80x9cWet strength agentsxe2x80x9d. An integral part of the invention is the material used to immobilize the bonds between the fibers in the wet state. Typically the means by which fibers are held together in paper and tissue products involve hydrogen and sometimes combinations of hydrogen bonds and covalent and/or ionic bonds. In the present invention, it is important to provide a material that will allow bonding of fibers in such a way as to immobilize the fiber to fiber bond points and make them resistant to disruption in the wet state. In this instance the wet state usually will mean when the product is exposed to water or other aqueous solutions, but could also mean exposure to body fluids such as urine, blood, mucus, menses, lymph and other body exudates.
There are a number of materials commonly used in the paper industry to impart wet strength to paper and board that are applicable to this invention. These materials are known in the art as xe2x80x9cwet strength agentsxe2x80x9d and are commercially available from a wide variety of sources. Any material that when added to a paper web or sheet results in providing the sheet with a wet geometric tensile strength:dry geometric tensile strength ratio in excess of 0.1 will, for purposes of this invention, be termed a wet strength agent. Typically these materials are termed either as permanent wet strength agents or as xe2x80x9ctemporaryxe2x80x9d wet strength agents. For the purposes of differentiating permanent from temporary wet strength, permanent will be defined as those resins which, when incorporated into paper or tissue products, will provide a product that retains more than 50% of its original wet strength after exposure to water for a period of at least five minutes. Temporary wet strength agents are those which show less than 50% of their original wet strength after exposure to water for five minutes. Both classes of material find application in the present invention. The amount of wet strength agent added to the pulp fibers can be at least about 0.1 dry weight percent, more specifically about 0.2 dry weight percent or greater, and still more specifically from about 0.1 to about 3 dry weight percent based on the dry weight of the fibers.
Permanent wet strength agents will provide a more or less long-term wet resilience to the structure. This type of structure would find application in products that would require long-term wet resilience such as in paper towels and in many absorbent consumer products. In contrast, the temporary wet strength agents would provide structures that had low density and high resilience, but would not provide a structure that had long-term resistance to exposure to water or body fluids. While the structure would have good integrity initially, after a period of time the structure would begin to lose its wet resilience. This property can be used to some advantage in providing materials that are highly absorbent when initially wet, but which after a period of time lose their integrity. This property could be used in providing xe2x80x9cflushablexe2x80x9d products. The mechanism by which the wet strength is generated has little influence on the products of this invention as long as the essential property of generating water-resistant bonding at the fiber/fiber bond points is obtained.
The permanent wet strength agents that are of utility in the present invention are typically water soluble, cationic oligomeric or polymeric resins that are capable of either crosslinking with themselves (homocrosslinking) or with the cellulose or other constituent of the wood fiber. The most widely-used materials for this purpose are the class of polymer known as polyamide-polyamine-epichlorohydrin (PAE) type resins. These materials have been described in patents issued to Keim (U.S. Pat. Nos. 3,700,623 and 3,772,076) and are sold by Hercules, Inc., Wilmington, Del., as Kymene 557H. Related materials are marketed by Henkel Chemical Co., Charlotte, N.C. and Georgia-Pacific Resins, Inc., Atlanta, Ga.
Polyamide-epichlorohydrin resins are also useful as bonding resins in this invention. Materials developed by Monsanto and marketed under the Santo Res label are base-activated polyamide-epichlorohydrin resins that can be used in the present invention. These materials are described in patents issued to Petrovich (U.S. Pat. No. 3,885,158; U.S. Pat. No. 3,899,388; U.S. Pat. No. 4,129,528 and U.S. Pat. No. 4,147,586) and van Eenam (U.S. Pat. No. 4,222,921). Although they are not as commonly used in consumer products, polyethylenimine resins are also suitable for immobilizing the bond points in the products of this invention. Another class of permanent-type wet strength agents are exemplified by the aminoplast resins obtained by reaction of formaldehyde with melamine or urea.
The temporary wet strength resins that can be used in connection with this invention include, but are not limited to, those resins that have been developed by American Cyanamid and are marketed under the name Parez 631 NC (now available from Cytec Industries, West Paterson, N.J.). This and similar resins are described in U.S. Pat. No. 3,556,932 to Coscia et al. and U.S. Pat. No. 3,556,933 to Williams et al. Other temporary wet strength agents that should find application in this invention include modified starches such as those available from National Starch and marketed as Co-Bond 1000. It is believed that these and related starches are covered by U.S. Pat. No. 4,675,394 to Solarek et al. Derivatized dialdehyde starches, such as described in Japanese Kokai Tokkyo Koho JP 03,185,197, should also find application as useful materials for providing temporary wet strength. It is also expected that other temporary wet strength materials such as those described in U.S. Pat. No. 4,981,557; U.S. Pat. No. 5,008,344 and U.S. Pat. No. 5,085,736 to Bjorkquist would be of use in this invention. With respect to the classes and the types of wet strength resins listed, it should be understood that this listing is simply to provide examples and that this is neither meant to exclude other types of wet strength resins, nor is it meant to limit the scope of this invention.
Although wet strength agents as described above find particular advantage for use in connection with in this invention, other types of bonding agents can also be used to provide the necessary wet resiliency. They can be applied at the wet end or applied by spraying or printing, etc. after the web is formed or after it is dried.
As used herein, the xe2x80x9cwet:dry ratioxe2x80x9d is the ratio of the geometric mean wet tensile strength divided by the geometric mean dry tensile strength. Geometric mean tensile strength (GMT) is the square root of the product of the machine direction tensile strength and the cross-machine direction tensile strength of the web. Unless otherwise indicated, the term xe2x80x9ctensile strengthxe2x80x9d means xe2x80x9cgeometric mean tensile strength.xe2x80x9d The webs of this invention have a wet:dry ratio of about 0.1 or greater, more specifically about 0.15 or greater, more specifically about 0.2 or greater, still more specifically about 0.3 or greater, still more specifically about 0.4 or greater, and still more specifically from about 0.2 to about 0.6. Tensile strengths can be measured using an Instron tensile tester using a 3 inches jaw width, a jaw span of 4 inches, and a crosshead speed of 10 inches per minute after maintaining the sample under TAPPI conditions for 4 hours before testing. The webs of this invention also preferably have a minimum absolute ratio of dry tensile strength to basis weight of 10 grams/gsm, preferably 15 grams/gsm, more preferably 20 grams/gsm, more preferably 30 grams/gsm, and still more preferably 40 grams/gsm and preferably from about 20 to about 100 grams/gsm. The webs of this invention also preferably have a minimum absolute ratio of wet tensile strength to basis weight of about 1 gram/gsm, preferably about 2 grams/gsm, more preferably about 5 grams/gsm, more preferably about 10 grams/gsm and still more preferably about 20 grams/gsm and preferably from about 15 to about 50 grams/gsm.
xe2x80x9cOverall Surface Deathxe2x80x9d. A three-dimensional basesheet or web is a sheet with significant variation in surface elevation due to the intrinsic structure of the sheet itself. As used herein, this elevation difference is expressed as the xe2x80x9cOverall Surface Depth.xe2x80x9d The webs of this invention possess three-dimensionality and have an Overall Surface Depth of about 0.1 mm. or greater, more specifically about 0.3 mm. or greater, still more specifically about 0.4 mm. or greater, still more specifically about 0.5 mm. or greater, and still more specifically from about 0.4 to about 0.8 mm.
The three-dimensional structure of a largely planar sheet can be described in terms of its surface topography. Rather than presenting a nearly flat surface. as is typical of conventional paper, the molded sheets of the present invention have significant topographical structures that derive in part from the use of sculptured through-drying fabrics such as those taught by Chiu et al. in U.S. Pat. No. 5,429,686, xe2x80x9cApparatus for Making Soft Tissue Products,xe2x80x9d issued Jul. 4, 1995, which is hereby incorporated by reference. The resulting paper surface topography typically comprises a regular repeating unit cell that is typically a parallelogram with sides between 2 and 20 mm in length. It is important that these three-dimensional structures be created by molding the moist sheet or be created prior to drying, rather than by creping or embossing or other operations after the sheet has been dried. In this manner, the three-dimensional structure is more likely to be well-retained upon wetting, helping to provide high wet resiliency and to promote good in-plane permeability.
In addition to the regular geometrical structure imparted by the sculptured fabrics and other fabrics used in creating a sheet, additional fine structure, with an in-plane length scale less than about 1 mm, can be present in the sheet. Such a fine structure can stem from microfolds created during differential velocity transfer of the web from one fabric or wire to another prior to drying. Some of the materials of the present invention, for example, appear to have fine structure with a fine surface depth of 0.1 mm or greater, and sometimes 0.2 mm or greater, when height profiles are measured using a commercial moire interferometer system. These fine peaks have a typical half-width less than 1 mm. The fine structure from differential velocity transfer and other treatments may be useful in providing additional softness, flexibility, and bulk. Measurement of the surface structures is described below.
An especially suitable method for measurement of Overall Surface Depth is moirxc3xa9 interferometry, which permits accurate measurement without deformation of the surface. For reference to the materials of the present invention, surface topography should be measured using a computer-controlled white-light field-shifted moire interferometer with about a 38 mm field of view. The principles of a useful implementation of such a system are described in Bieman et al. (L. Bieman, K. Harding, and A. Boehnlein, xe2x80x9cAbsolute Measurement Using Field-Shifted Moirxc3xa9,xe2x80x9d SPIE Optical Conference Proceedings, Vol. 1614, pp. 259-264, 1991). A suitable commercial instrument for moirxc3xa9 interferometry is the CADEYES(copyright) interferometer produced by Medar, Inc. (Farmington Hills, Mich.), constructed for a 38-mm field-of-view (a field of view within the range of 37 to 39 5 mm is adequate). The CADEYES(copyright) system uses white light which is projected through a diffraction grid to project fine black lines onto the sample surface. The surface is viewed through a similar diffraction grid, creating moirxc3xa9 fringes that are viewed by a CCD camera. Suitable lenses and a stepper motor adjust the optical configuration for field shifting (a technique described below). A video processor sends captured fringe images to a PC computer for processing, allowing details of surface height to be back-calculated from the fringe patterns viewed by the video camera.
In the CADEYES moirxc3xa9 interferometry system, each pixel in the CCD video image is said to belong to a moirxc3xa9 fringe that is associated with a particular height range. The method of field-shifting, as described by Bieman et al. (L. Bieman, K. Harding, and A. Boehnlein, xe2x80x9cAbsolute Measurement Using Field-Shifted Moirxc3xa9,xe2x80x9d SPIE Optical Conference Proceedings, Vol. 1614, pp. 259-264, 1991) and as originally patented by Boehnlein (U.S. Pat. No. 5,069,548, herein incorporated by reference), is used to identify the fringe number for each point in the video image (indicating which fringe a point belongs to). The fringe number is needed to determine the absolute height at the measurement point relative to a reference plane. A field-shifting technique (sometimes termed phase-shifting in the art) is also used for sub-fringe analysis (accurate determination of the height of the measurement point within the height range occupied by its fringe). These field-shifting methods coupled with a camera-based interferometry approach allows accurate and rapid absolute height measurement, permitting measurement to be made in spite of possible height discontinuities in the surface. The technique allows absolute height of each of the roughly 250,000 discrete points (pixels) on the sample surface to be obtained, if suitable optics, video hardware, data acquisition equipment, and software are used that incorporates the principles of moirxc3xa9 interferometry with field-shifting. Each point measured has a resolution of approximately 1.5 microns in its height measurement.
The computerized interferometer system is used to acquire topographical data and then to generate a grayscale image of the topographical data, said image to be hereinafter called xe2x80x9cthe height map.xe2x80x9d The height map is displayed on a computer monitor, typically in 256 shades of gray and is quantitatively based on the topographical data obtained for the sample being measured. The resulting height map for the 38-mm square measurement area should contain approximately 250,000 data points corresponding to approximately 500 pixels in both the horizontal and vertical directions of the displayed height map. The pixel dimensions of the height map are based on a 512xc3x97512 CCD camera which provides images of moire patterns on the sample which can be analyzed by computer software. Each pixel in the height map represents a height measurement at the corresponding x- and y-location on the sample. In the recommended system, each pixel has a width of approximately 70 microns, i.e. represents a region on the sample surface about 70 microns long in both orthogonal in-plane directions). This level of resolution prevents single fibers projecting above the surface from having a significant effect on the surface height measurement. The z-direction height measurement must have a nominal accuracy of less than 2 microns and a z-direction range of at least 1.5 mm. (For further background on the measurement method, see the CADEYES Product Guide, Medar, Inc., Farmington Hills, Mich., 1994, or other CADEYES manuals and publications of Medar, Inc.)
The CADEYES system can measure up to 8 moirxc3xa9 fringes, with each fringe being divided into 256 depth counts (sub-fringe height increments, the smallest resolvable height difference). There will be 2048 height counts over the measurement range. This determines the total z-direction range, which is approximately 3 mm in the 38-mm field-of-view instrument. If the height variation in the field of view covers more than eight fringes, a wrap-around effect occurs, in which the ninth fringe is labeled as if it were the first fringe and the tenth fringe is labeled as the second, etc. In other words, the measured height will be shifted by 2048 depth counts. Accurate measurement is limited to the main field of 8 fringes.
The moirxc3xa9 interferometer system, once installed and factory calibrated to provide the accuracy and z-direction range stated above, can provide accurate topographical data for materials such as paper towels. (Those skilled in the art may confirm the accuracy of factory calibration by performing measurements on surfaces with known dimensions.) Tests are performed in a room under Tappi conditions (73xc2x0 F., 50% relative humidity). The sample must be placed flat on a surface lying aligned or nearly aligned with the measurement plane of the instrument and should be at such a height that both the lowest and highest regions of interest are within the measurement region of the instrument.
Once properly placed, data acquisition is initiated using Medar""s PC software and a height map of 250,000 data points is acquired and displayed, typically within 30 seconds from the time data acquisition was initiated. (Using the CADEYES(copyright) system, the xe2x80x9ccontrast threshold levelxe2x80x9d for noise rejection is set to 1, providing some noise rejection without excessive rejection of data points.) Data reduction and display are achieved using CADEYES(copyright) software for PCs, which incorporates a customizable interface based on Microsoft Visual Basic Professional for Windows (version 3.0). The Visual Basic interface allows users to add custom analysis tools.
The height map of the topographical data can then be used by those skilled in the art to identify characteristic unit cell structures (in the case of structures created by fabric patterns; these are typically parallelograms arranged like tiles to cover a larger two-dimensional area) and to measure the typical peak to valley depth of such structures. A simple method of doing this is to extract two-dimensional height profiles from lines drawn on the topographical height map which pass through the highest and lowest areas of the unit cells. These height profiles can then be analyzed for the peak to valley distance, if the profiles are taken from a sheet or portion of the sheet that was lying relatively flat when measured. To eliminate the effect of occasional optical noise and possible outliers, the highest 10% and the lowest 10% of the profile should be excluded, and the height range of the remaining points is taken as the surface depth. Technically, the procedure requires calculating the variable which we term xe2x80x9cP10,xe2x80x9d defined at the height difference between the 10% and 90% material lines, with the concept of material lines being well known in the art, as explained by L. Mummery, in Surface Texture Analysis: The Handbook, Hommelwerke GmbH, Mxc3xchlhausen, Germany, 1990. In this approach, the surface is viewed as a transition from air to material. For a given profile, taken from a flat-lying sheet, the greatest height at which the surface beginsxe2x80x94the height of the highest peakxe2x80x94is the elevation of the xe2x80x9c0% reference linexe2x80x9d or the xe2x80x9c0% material line,xe2x80x9d meaning that 0% of the length of the horizontal line at that height is occupied by material. Along the horizontal line passing through the lowest point of the profile, 100% of the line is occupied by material, making that line the xe2x80x9c100% material line.xe2x80x9d In between the 0% and 100% material lines (between the maximum and minimum points of the profile), the fraction of horizontal line length occupied by material will increase monotonically as the line elevation is decreased. The material ratio curve gives the relationship between material fraction along a horizontal line passing through the profile and the height of the line; this relationship is sketched in FIG. 2. The material ratio curve is also the cumulative height distribution of a profile. (A more accurate term might be xe2x80x9cmaterial fraction curve.xe2x80x9d)
Once the material ratio curve is established, one can use it to define a characteristic peak height of the profile. The P10 xe2x80x9ctypical peak-to-valley heightxe2x80x9d parameter is defined as the difference between the heights of the 10% and 90% material lines. This parameter is relatively robust in that outliers or unusual excursions from the typical profile structure have little influence on the P10 height. The units of P10 are mm. The Overall Surface Depth of a material is reported as the P10 surface depth value for profile lines encompassing the height extremes of the typical unit cell of that surface. xe2x80x9cFine surface depthxe2x80x9d is the P10 value for a profile taken along a plateau region of the surface which is relatively uniform in height relative to profiles encompassing a maxima and minima of the unit cells. Measurements are reported for the most textured side of the materials of the present invention, which is typically the side that was in contact with the through-drying fabric when air flow is toward the through-dryer. FIGS. 3A, 3B and 3C show typical profiles from a sample of SURPASS, which is a commercial uncreped, through-dried material made with secondary fibers. The Overall Surface Depth is seen to be about 0.3 mm. Typical fine elements have a fine surface depth less than 0.15 mm. FIGS. 4A, 4B and 4C present profiles from Sample U2 of the present invention, described hereafter in the Examples. The Overall Surface Depth is over 0.4 mm, and the fine structure has a surface depth of about 0.3 mm. FIG. 5 represents a profile of Sample U8 of the present invention, having an Overall Surface Depth of about 0.5 mm.
Overall Surface Depth is intended to examine the topography produced in the basesheet, especially those features created in the sheet prior to and during drying processes, but is intended to exclude xe2x80x9cartificiallyxe2x80x9d created large-scale topography from dry converting operations such as embossing, perforating, pleating, etc. Therefore, the profiles examined should be taken from unembossed regions if the sheet has been embossed, or should be measured on an unembossed sheet. Overall Surface Depth measurements should exclude large-scale structures such as pleats or folds which do not reflect the three-dimensional nature of the original basesheet itself. It is recognized that sheet topography may be reduced by calendering and other operations which affect the entire basesheet. Overall Surface Depth measurement can be appropriately performed on a calendered sheet.
The xe2x80x9cWet Wrinkle Recovery Testxe2x80x9d is used to quantify wet bending resiliency. It is a slight modification of AATCC Test Method 66-1990 taken from the Technical Manual of the American Association of Textile Chemists and Colorists (1992), page 99. The modification is to first wet the samples before carrying out the method. This is done by soaking the samples in water containing 0.01 percent TRITON X-100 wetting agent (Rohm and Haas) for five minutes before testing. Sample preparation is carried out at 73xc2x0 F. and 50 percent relative humidity. The sample is gently removed from the water with a tweezers, drained by pressing between two pieces of blotter paper with 325 grams of weight, and placed in the sample holder to be tested as with the dry wrinkle recovery test method. The test measures the highest recovery angle of the sample being tested (in any direction, including the machine direction and the cross-machine direction), with 180xc2x0 representing total recovery. The Wet Wrinkle Recovery, expressed as a percent recovery, is the measured recovery angle divided by 180xc2x0, multiplied by 100. Absorbent structures of this invention can exhibit a Wet Wrinkle Recovery of about 60 percent or greater, more specifically about 70 percent or greater, and still more specifically about 80 percent or greater.
xe2x80x9cWet compressive resiliencyxe2x80x9d of the new materials is defined by several parameters and can be demonstrated using a materials property procedure that encompasses both wet and dry characteristics. A programmable strength measurement device is used in compression mode to impart a specified series of compression cycles to an initially dry, conditioned sample, after which the sample is carefully moistened in a specified manner and subjected to the same sequence of compression cycles. While the comparison of wet and dry properties is of general interest, the most important information from this test concerns the wet properties. The initial testing of the dry sample can be viewed as a conditioning step. The test sequence begins with compression of the dry sample to 0.025 psi to obtain an initial thickness (cycle A), then two repetitions of loading up to 2 psi followed by unloading (cycles B and C). Finally, the sample is again compressed to 0.025 psi to obtain a final thickness (cycle D). (Details of the procedure, including compression speeds, are given below). Following the treatment of the dry sample, moisture is applied uniformly to the sample using a fine mist of deionized water to bring the moisture ratio (g water/g dry fiber) to approximately 1.1. This is done by applying 95-110% added moisture, based on the conditioned sample mass. This puts typical cellulosic materials in a moisture range where physical properties are relatively insensitive to moisture content (e.g., the sensitivity is much less than it is for moisture ratios less than 70%). The moistened sample is then placed in the test device and the compression cycles are repeated.
Three measures of wet resiliency are considered which are relatively insensitive to the number of sample layers used in the stack. The first measure is the bulk of the wet sample at 2 psi. This is referred to as the xe2x80x9cWet Compressed Bulkxe2x80x9d (WCB). The second measure is termed xe2x80x9cWet Springback Ratio (WS)xe2x80x9d, which is the ratio of the moist sample thickness at 0.025 psi at the end of the compression test (cycle D) to the thickness of the moist sample at 0.025 psi measured at the beginning of the test (cycle A). The third measure is the xe2x80x9cLoading Energy Ratioxe2x80x9d (LER), which is the ratio of loading energy in the second compression to 2 psi (cycle C) to that of the first compression to 2 psi (cycle B) during the sequence described above, for a wetted sample. The final wet bulk measured at the end of the test (at 0.025 psi) is termed the xe2x80x9cfinal bulkxe2x80x9d or xe2x80x9cFBxe2x80x9d value. When load is plotted as a function of thickness, loading energy is the area under the curve as the sample goes from an unloaded state to the peak load of that cycle. For a purely elastic material, the springback and loading energy ratio would be unity. We have found that the three measures described here are relatively independent of the number of layers in the stack and serve as useful measures of wet resiliency. Also referred to herein is the xe2x80x9cCompression Ratioxe2x80x9d, which is defined as the ratio of moistened sample thickness at peak load in the first compression cycle to 2 psi to the initial moistened thickness at 0.025 psi.
In carrying out the foregoing measurements of the wet compressive resiliency, samples should be conditioned for at least 24 hours under TAPPI conditions (50% RH, 73xc2x0 F.). Specimens are die cut to 2.5xe2x80x3xc3x972.5xe2x80x3 squares. Conditioned sample weight should be near 0.4 g, if possible, and within the range of 0.25 to 0.6 g for meaningful comparisons. The target mass of 0.4 to 0.5 gram is achieved by using a stack of 2 or more sheets if the sheet basis weight is less than 65 gsm. For example, for nominal 30 gsm sheets, a stack of 3 sheets will generally be near 0.4 g total mass. Three sheets are preferred for 40 gsm sheets, while 2 sheets should be used for 60 gsm sheets.
Compression measurements are performed using an Instron 4502 Universal Testing Machine interfaced with a 286 PC computer running Instron Series XII software (1989 issue) and Version 2 firmware. The standard xe2x80x9c286 computerxe2x80x9d referred to has an 80286 processor with a 12 MHz clock speed. The particular computer used was a Compaq DeskPro 286e with an 80287 math coprocessor and a VGA video adapter. A 1 kN load cell is used with 2.25xe2x80x3 diameter circular platens for sample compression. The lower platen has a ball bearing assembly to allow exact alignment of the platens. The lower platen is locked in place while under load (30-100 lbf) by the upper platen to ensure parallel surfaces. The upper platen must also be locked in place with the standard ring nut to eliminate play in the upper platen as load is applied.
Following at least one hour of warm-up after start-up, the instrument control panel is used to set the extensionometer to zero distance while the platens are in contact (at a load of 10-30 lb). With the upper platen freely suspended, the calibrated load cell is balanced to give a zero reading. The extensionometer and load cell should be periodically checked to prevent baseline drift (shirting of the zero points). Measurements must be performed in a controlled humidity and temperature environment, according to TAPPI specifications (50%xc2x12% rh and 73xc2x0 F.). The upper platen is then raised to a height of 0.2 in. and control of the Instron is transferred to the computer.
Using the Instron Series XII Cyclic Test software with a 286 computer, an instrument sequence is established with 7 markers (discrete events) composed of 3 cyclic blocks (instructions sets) in the following order:
Marker 1: Block 1
Marker 2: Block 2
Marker 3: Block 3
Marker 4: Block 2
Marker 5: Block 3
Marker 6: Block 1
Marker 7: Block 3.
Block 1 instructs the crosshead to descend at 1.5 in./min. until a load of 0.1 lb. is applied (the Instron setting is xe2x88x920.1 lb., since compression is defined as negative force). Control is by displacement. When the targeted load is reached, the applied load is reduced to zero.
Block 2 directs that the crosshead range from an applied load of 0.05 lb. to a peak of 8 lb. then back to 0.05 lb. at a speed of 0.4 in./min. Using the Instron software, the control mode is displacement, the limit type is load, the first level is xe2x88x920.05 lb., the second level is xe2x88x928 lb., the dwell time is 0 sec., and the number of transitions is 2 (compression, then relaxation); xe2x80x9cno actionxe2x80x9d is specified for the end of the block.
Block 3 uses displacement control and limit type to simply raise the crosshead to 0.2 in. at a speed of 4 in./min., with 0 dwell time. Other Instron software settings are 0 in first level, 0.2 in second level, 1 transition, and xe2x80x9cno actionxe2x80x9d at the end of the block.
When executed in the order given above (Markers 1-7), the Instron sequence compresses the sample to 0.025 psi (0.1 lbf), relaxes, then compresses to 2 psi (8 lbs.), followed by decompression and a crosshead rise to 0.2 in., then compress the sample again to 2 psi, relaxes, lifts the crosshead to 0.2 in., compresses again to 0.025 psi (0.1 lbf, and then raises the crosshead. Data logging should be performed at intervals no greater than every 0.02xe2x80x3 or 0.4 lb. (whichever comes first) for Block 2 and for intervals no greater than 0.01 lb. for Block 1. Preferably, data logging is performed every 0.004 lb. in Block 1 and every 0.05 lb. or 0.005 in. (whichever comes first) in Block 2.
The results output of the Series XII software is set to provide extension (thickness) at peak loads for Markers 1, 2, 4 and 6 (at each 0.025 and 2.0 psi peak load), the loading energy for Markers 2 and 4 (the two compressions to 2.0 psi previously termed cycles B and C, respectively), the ratio of the two loading energies (second cycle/first cycle), and the ratio of final thickness to initial thickness (ratio of thickness at last to first 0.025 psi compression). Load versus thickness results are plotted on the screen during execution of Blocks 1 and 2.
In performing a measurement, the dry, conditioned sample is centered on the lower platen and the test is initiated. Following completion of the sequence, the sample is immediately removed and moisture (deionized water at 72-73xc2x0 F.) is applied. Moisture is applied uniformly with a fine mist to reach a moist sample mass of approximately 2.0 times the initial sample mass (95-110% added moisture is applied, preferably 100% added moisture, based on conditioned sample mass; this level of moisture should yield an absolute moisture ratio of about 1.1 g. water/g. oven dry fiberxe2x80x94with oven dry referring to drying for at least 30 minutes in an oven at 105xc2x0 C.). (For the uncreped throughdried materials of this invention, the moisture ratio could be within the range of 1.05 to 1.7 without significantly affecting the results). The mist should be applied uniformly to separated sheets (for stacks of more than 1 sheet), with spray applied to both front and back of each sheet to ensure uniform moisture application. This can be achieved using a conventional plastic spray bottle, with a container or other barrier blocking most of the spray, allowing only about the upper 10-20% of the spray envelopexe2x80x94a fine mistxe2x80x94to approach the sample. The spray source should be at least 10xe2x80x3 away from the sample during spray application. In general, care must be applied to ensure that the sample is uniformly moistened by a fine spray. The sample must be weighed several times during the process of applying moisture to reach the targeted moisture content. No more than three minutes should elapse between the completion of the compression test on the dry sample and the completion of moisture application. Allow 45-60 seconds from the final application of spray to the beginning of the subsequent compression test to provide time for internal wicking and absorption of the spray. Between three and four minutes will elapse between the completion of the dry compression sequence and initiation of the wet compression sequence.
Once the desired mass range has been reached, as indicated by a digital balance, the sample is centered on the lower Instron platen and the test sequence is initiated. Following the measurement, the sample is placed in a 105xc2x0 C. oven for drying, and the oven dry weight will be recorded later (sample should be allowed to dry for 30-60 minutes, after which the dry weight is measured).
Note that creep recovery can occur between the two compression cycles to 2 psi, so the time between the cycles may be important. For the instrument settings used in these Instron tests, there is roughly a 30 second period (typically xc2x14 sec.) between the beginning of compression during the two cycles to 2 psi. The beginning of compression is defined as the point at which the load cell reading exceeds 0.03 lb. Likewise, there is a 5-8 second interval between the beginning of compression in the first thickness measurement (ramp to 0.025 psi) and the beginning of the subsequent compression cycle to 2 psi. The interval between the beginning of the second compression cycle to 2 psi and the beginning of compression for the final thickness measurement is approximately 20 seconds.
The utility of a web or absorbent structure having a high Wet Compressed Bulk (WCB) value is obvious, for a wet material which can maintain high bulk under compression can maintain higher fluid capacity and is less likely to allow fluid to be squeezed out when it is compressed.
High Wet Springback Ratio values are especially desirable because a wet material that springs back after compression can maintain high pore volume for effective intake and distribution of subsequent insults of fluid, and such a material can regain fluid during its expansion which may have been expelled during compression. In diapers, for example, a wet region may be momentarily compressed by body motion or changes in body position. If the material is unable to regain its bulk when the compressive force is released, its effectiveness for handling fluid is reduced.
High Loading Energy Ratio values in a material are also useful, for such a material continues to resist compression (LER is based on a measure of the energy required to compress a sample) at loads less than the peak load of 2 psi, even after it has been heavily compressed once. Maintaining such wet elastic properties is believed to contribute to the feel of the material when used in absorbent articles, and may help maintain the fit of the absorbent article against the wearer""s body, in addition to the general advantages accrued when a structure can maintain its pore volume when wet.
The webs of this invention can exhibit one or more of the foregoing properties. More specifically, the webs of this invention can have a Wet Compressed Bulk of about 6 cubic centimeters per gram or greater, more specifically about 7 cubic centimeters per gram or greater, more specifically about 8 cubic centimeters per gram or greater, and still more specifically from about 8 to about 13 cubic centimeters per gram. The Compression Ratio can be about 0.7 or less, more specifically about 0.6 or less, still more specifically about 0.5 or less, and still more specifically from 0.4 to about 0.7. Also, they can have a Wet Springback Ratio of about 0.75 or greater, more specifically about 0.85 or greater, more specifically about 0.90 or greater, and still more specifically from about 0.8 to about 0.93. The Loading Energy Ratio can be about 0.7 or greater, more specifically about 0.8 or greater, and still more specifically from about 0.7 to about 0.9.
xe2x80x9cIn-Plane Permeabilityxe2x80x9d. An important property of porous media, particularly for absorbent products, is the permeability to liquid flow. The complex, interconnected pathways between the solid particles and boundaries of a porous media provide routes for fluid flow which may offer significant flow resistance due to the narrowness of the channels and the tortuosity of the pathways.
For paper, permeability is commonly expressed in terms of gas flow rates through a sheet. This practice is useful for comparing similar sheets, but does not truly characterize the interaction of flowing fluid with the porous structure and provides no direct information about flow in a wet sheet. The standard engineering definition of permeability provides a more useful parameter, though one less easily measured. The standard definition is based on Darcy""s law (see F. A. L. Dullien, Porous Media: Fluid Transport and Pore Structure, Academic Press, New York, 1979), which, for one-dimensional flow, states that the velocity of fluid flow through a saturated porous medium is directly proportional to the pressure gradient:                     V        =                              K            μ                    ⁢                      xe2x80x83                    ⁢                                    Δ              ⁢                              xe2x80x83                            ⁢              P                        L                                              (        1        )            
where V is the superficial velocity (flow rate divided by area), K is the permeability, xcexc is the fluid viscosity, and xcex94P is the pressure drop in the flow direction across a distance L. The units of K are m2. In Equation (1), the permeability is an empirical proportionality parameter linking fluid velocity to pressure drop and viscosity. For a homogeneous medium, K is not a function of xcex94P, sample length, or viscosity, but is an intrinsic parameter describing the flow resistance of the medium. In a compressible medium, permeability will be a function of the degree of compression. Darcian permeability is a fundamental parameter for processes involving fluid flow in fibrous webs.
Darcian permeability has units of area (m2) and for simple uniform cylindrical pores is proportional to the cross sectional area of a single pore. However, the permeability of most real materials cannot be predicted from an optical assessment of pore size. Permeability is determined not only by pore size, but also pore orientation, tortuosity, and interconnectedness. Large pores in the body of an object may be inaccessible to fluid flow or accessible only through minute pores offering high flow resistance. Even with a full three-dimensional description of the pore space of a material from x-ray tomography or other imaging techniques, it is difficult to predict or calculate the permeability. Permeability and pore size determinations are related but distinct pieces of information about a material. For example, a sheet of metal with discreet, nonoverlapping holes punched in it may have very large pores (the holes), while still having negligible In-Plane Permeability. Swiss cheese has many large pores, but typically has negligible permeability in any direction unless sliced so thin that individual holes can extend from one face to the other of the cheese sample.
Most studies of permeability in paper have focused on flow in the z-direction (normal to the plane of the sheet), which is of practical importance in wet pressing and other unit operations. However, paper is an anisotropic material (for example, see E. L. Back, xe2x80x9cThe Pore Anisotropy of Paper Products and Fibre Building Boards,xe2x80x9d Svensk Papperstidning, 69: 219 (1966)), meaning that fluid flow properties are a function of direction. In this case, different flow directions will appear to have different apparent permeabilities. The many possibilities of flow direction and pressure gradients in such a medium can be encompassed with a multidimensional form of Darcy""s law,                                           v            _                    =                                                    -                                                      K                    _                                    _                                            ·                              ∇                P                                      μ                          ,                            (        2        )            
where {overscore (v)} is the superficial velocity vector (volumetric flow rate divided by cross-sectional area of the flow), xcexc is the viscosity of the fluid, {double overscore (K)} is a second-order tensor and ∇P is the pressure gradient. If a Cartesian coordinate system is chosen to correspond with the principal flow directions of the porous medium, then the permeability tensor becomes a diagonal matrix (see Jacob Bear, xe2x80x9cDynamics of Fluids in Porous Media.,xe2x80x9d American Elsevier, New York, N.Y., 1972, pp. 136-151):                                                         K              _                        _                    =                      [                                                                                K                    x                                                                    0                                                  0                                                                              0                                                                      K                    y                                                                    0                                                                              0                                                  0                                                                      K                    z                                                                        ]                          ,                            (        3        )            
where Kx, Ky, and Kz are the principal permeability components in the x-, y-, and z-directions, respectively. In paper, these directions will generally correspond to the cross-direction (taken here as y) and the machine-direction (taken as x, the direction of maximum In-Plane Permeability) in the plane, and the transverse or thickness direction (z). Thus, the anisotropic permeability of typical machine-made paper can be characterized with three permeability parameters, one for the machine-directicn, one for the cross-direction, and one for the z-direction. (In some cases, as when there are unbalanced flows in the headbox of the paper machine, the direction of maximum permeability may be slightly off from the machine direction; the direction of maximum In-Plane Permeability and the direction orthogonal to that should be used for the x- and y-directions, respectively, in that case.) In handsheets, there may be no preferential direction of orientation for fibers lying in the plane, so the x- and y-direction permeability values should be equal (in other words, such a sheet is isotropic in the plane).
In spite of the past focus on z-direction permeability in paper, In-Plane Permeability (both Kx and Ky are in-plane factors) is important in a variety of applications, especially in absorbent articles. Body fluids or other liquids flowing into the absorbent article usually enter the article in a narrow, localized region. Efficient use of the absorbent medium requires that the incoming fluid be distributed laterally through in-plane flow in the absorbent article, otherwise the local capacity of the article to handle the incoming liquid may be overwhelmed resulting in leakage and poor utilization of the absorbent core. The ability of fluid to flow in the plane of the article is a function of the driving force for fluid flow, which can be a combination of capillary wicking and hydraulic pressure from fluid source, and of the ability of the porous medium to conduct flow, which is described in large part by the Darcian permeability of the material. Two-phase flow and non-Newtonian liquids or suspensions complicate the physics, but the in-plane permeability of the porous medium is a critical factor for rapid in-plane distribution of liquid insults. Especially in the case of urine management, where liquid flow rates may occur far in excess of the ability of capillary forces, high In-Plane Permeability is needed in the intake layer to allow the fluid to be distributed laterally rather than to leak.
While many past studies of liquid permeability in paper focused exclusively on measuring Kz for z-direction flow, more recently, methods have been taught for measuring permeability in the plane of a paper sheet. J. D. Lindsay and P. H. Brady teach methods for in-plane and z-direction permeability measurements of saturated paper in xe2x80x9cStudies of Anisotropic Permeability with Applications to Water Removal in Fibrous Webs: Part I,xe2x80x9d Tappi J., 76(9): 119-127 (1993) and xe2x80x9cStudies of Anisotropic Permeability with Applications to Water Removal in Fibrous Webs: Part II,xe2x80x9d Tappi J., 76(11): 167-174 (1993). Related methods have been published by K. L. Adams, B. Miller, and L. Rebenfeld in xe2x80x9cForced In-Plane Flow of an Epoxy Resin in Fibrous Networks,xe2x80x9d Polymer Engineering and Science, 26(20): 1434-1441 (1986); J. D. Lindsay in xe2x80x9cRelative Flow Porosity in Fibrous Media: Measurements and Analysis, Including Dispersion Effects,xe2x80x9d Tappi J., 77(6): 225-239 (June 1994); J. D. Lindsay and J. R. Wallin, xe2x80x9cCharacterization of In-Plane Flow in Paper,xe2x80x9d AIChE 1989 and 1990 Forest Products Symposium, Tappi Press, Atlanta, Ga. (1992), p.121; and D. H. Horstmann, J. D. Lindsay, and R. A. Stratton, xe2x80x9cUsing Edge-Flow Tests to Examine the In-Plane Anisotropic Permeability of Paper,xe2x80x9d Tappi J., 74(4): 241 (1991).
The basic method used in most of these publications is injection of fluid into the center of a paper disk that is constrained between two flat surfaces to force the fluid flow to be in the radial direction, proceeding from the injection point at the center of the disk to the outer edge of the disk. This is illustrated in FIG. 6, which depicts a sheet in which a central hole has been punched and into which fluid is injected by means of an injection port of the same size as the punched hole. For a liquid-saturated sheet of constant thickness subject to steady radial fluid flow in the manner described in the work of Lindsay and others, the equation relating average In-Plane Permeability to fluid flow is:                                                         K              r                        ≡                                                            K                  x                                +                                  K                  y                                            2                                =                                    Q              ⁢                              xe2x80x83                            ⁢                              μln                ⁡                                  (                                                            R                      o                                        /                                          R                      i                                                        )                                                                    2              ⁢              π              ⁢                              xe2x80x83                            ⁢                              L                p                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢              P                                      ,                            (        4        )            
where Ro is the radius of the paper disk, Ri is the radius of the central hole in the sample into which fluid is injected through an injection port; Lp is the thickness of the paper; xcex94P is the constant pressure above atmospheric pressure at which fluid is injected into the disk (the gauge pressure at the injection pore); Q is the volumetric flow rate of liquid, and Kr is the In-Plane Permeability, technically the average radial permeability, defined as the average of the two in-plane permeability components.
Details of the disk geometry used in the experimental work are shown in FIG. 7. The disk diameter is typically 5 inches, although in some cases, the maximum available sample size was 4.5 inches. The central inlet hole was consistently 0.375 inches (xe2x85x9c inch) and was created using a paper punch tool. The test apparatus for In-Plane Permeability measurements is depicted in FIGS. 8 and 9, which is identical in principle to the apparatus taught by Lindsay and Brady (op. cit.). Tubing connects water from a water reservoir to an injection port drilled into a 1-inch thick Plexiglas support plate. (The support plate is transparent to permit viewing of the wetted sample, especially in cases when an aqueous dye solution is injected into the sample. A mirror at a 45 degree angle below the support plate facilitates viewing and photography.) The water reservoir provides a nearly constant hydraulic head for fluid injection during the test. The volumetric flow rate is obtained by noting the change in water reservoir mass as a function of time, and converting the water mass flow rate to a volumetric flow rate. Vacuum-deaerated deionized water at room temperature is used.
In the apparatus of FIG. 8, a paper disk, cut to the dimensions shown in FIG. 7 (5-inch diameter and 0.375-inch central hole), is placed over the injection port (0.375 inches diameter also) and is then saturated with water. The fluid injection line and the injection port should be filled with water and efforts should be taken to avoid air bubbles being trapped in the sheet or in the injection area. To help eliminate air pockets, the sample should be bent gently in the center as it is placed on the wet support plate to initiate liquid contact in the center of the sample; the edges can then be lowered gradually to create a wedge-like motion of the liquid meniscus to sweep air bubbles out from under the sheet. Multi-ply stacks of sheets can be handled in the same way, although preliminary sample wetting may be needed to remove interply air bubbles. The goal in removing air bubbles is to reduce the flow blockage that trapped air bubbles can cause.
Once the wetted sample is in place, a cylindrical metal platen, 5-inches in diameter, is gently lowered on top of the sample to provide a constant compressive load and to provide a reference surface on its top for thickness measurement with displacement gauges. Three displacement gauges are used, spaced approximately evenly around the edge of the top of the metal cylinder, in order to measure the average thickness of the sheet. The sample thickness is taken as the average of the three displacement values relative to a zero point when no sample is present. A suitable thickness gauge is the Mitutoyo Digimatic Indicator, Model 543-525-1, with a 2-inch stroke (traveling distance of the contacting spindle) and a precision of 1 micrometer. The thickness gauges are rigidly mounted relative to the support plate. The contacting spindles of the thickness gauges can be raised and lowered (without changing the position of the body of the gauge) by use of a cable to provide clearance for moving the metal platen onto the sample. The small force applied by the thickness gauges should be added to the weight of the metal platen to obtain the total force applied to the sample; this force, when divided by the cross sectional area of the sample and platen, should be 0.8 psi.
A hydraulic head of 13 inches is used to drive the liquid flow. This head is achieved by placement of a water bottle, filled to a specified level, on a mass balance at a fixed height relative to the support plate on which the sample rests. As the sample is being placed on the support plate, the water reservoir is at such a height that the water level in the reservoir is nearly the same as (or slightly greater than) the support plate on which the sample rests. When the sample has been moistened and placed under the compressive load of the metal platen, the water reservoir is then raised and placed on a mass balance such that the water level is 13 inches above the support platen. A timer is activated and the water reservoir mass is recorded at 20 seconds or 30 seconds intervals for a least 90 seconds. The thickness readings of the three gauges is also recorded regularly during the test. To reduce creep, the saturated sample should be allowed to equilibrate under the compressive load for at least 30 seconds before the water bottle is raised and forced flow through the sample begins.
The change in water reservoir mass as a function of time gives the mass flow rate, which can easily be converted to a volumetric flow rate for use in Equation 4. Normal engineering principles should be used to ensure that the proper units (preferably SI units) are used in applying Equation 4.
In performing In-Plane Permeability measurements, it is important that the sample be uniformly compressed against the restraining surfaces to prevent large channels or openings that would provide paths of least resistance for substantial liquid flow that could bypass much of the sample itself. Ideally, the liquid will flow uniformly through the sample, and this can be ascertained by injecting dyed fluid into the sample and observing the shape of the dyed region through the transparent support plate. Injected dye should spread out uniformly from the injection point. In isotropic samples, the shape of the moving dye region should be nearly circular. In materials with in-plane anisotropy due to fiber orientation or small-scale structural orientation, the shape of the dye region should be oval or elliptical, and nearly symmetric about the injection point. A suitable dye for such tests is Versatint Purple II made by Milliken Chemical Corp. (Inman, S.C.). This is a fugitive dye that does not absorb onto cellulose, allowing for easy visualization of liquid flow through the fibrous medium.
In addition to specifying the average In-Plane Permeability, the ratio of the two in-plane components, or the in-plane anisotropy factor, xcex1, is also of interest. This factor is the ratio of the x-direction to y-direction permeability components, or                     α        =                                            K              x                                      K              y                                .                                    (        5        )            
Radial flow tests performed with dyed fluid can be used to determine the in-plane anisotropy factor, using an approximate solution to the fluid flow equations obtained by J. D. Lindsay in xe2x80x9cThe Anisotropic Permeability of Paper: Theory, Measurements, and Analytical Tools,xe2x80x9d IPC Technical Paper Series No. 289, Institute of Paper Science and Technology, Atlanta, Ga., July 1988, and applied in J. D. Lindsay, xe2x80x9cThe Anisotropic Permeability of Paper,xe2x80x9d Tappi J., 73(5): 223 (May 1990). To relate in-plane anisotropy to the shape of a moving dye boundary resulting from injection of dye into a disk saturated with clear water, Lindsay obtained an approximate analytical solution in polar coordinates by neglecting flow in the tangential or q-direction. In the selected polar coordinate frame, q=0 corresponds to the x-direction and q=p/2 to the y-direction. Let Rx and Ry be the radial locations of the dye boundary in the x- and y-directions, respectively. Then the approximate solution allows xcex1 to be determined from the geometry of the colored zone from the equation:                     α        =                              (                                          R                x                2                            -                              R                i                2                                      )                                (                                          R                y                2                            -                              R                i                2                                      )                                              (        6        )            
where Ri is the radius of the injection port at the center of the paper disk. This approximate solution was found to be highly accurate (when compared to numerical solutions of the flow problem) for the case of dye injected into a saturated disk and was also reasonably accurate for the case of dye injected into an initially dry disk.
For the In-Plane Permeability results to be a proper measure of the material in question, the permeability should reflect the resistance of the material itself and not the resistance of a large scale channel or void which has been created in some manner such as cutting, slitting, folding, pleating, etc. We therefore require that the material provide a radial flow uniformity that can be assessed by visualization of dye flow injected into the sample. Radial flow uniformity exists when dye injected into a dry sample with the previously described in-plane permeability apparatus results in a symmetric, roughly elliptical dye pattern. Such a dye pattern should yield a value of xcex1 (from Equation 6) less than 4 when Rx is taken to be the radial position of the portion of the dye boundary furthest from the inlet, and Ry is the radial position of the portion of the boundary closest to the inlet, at a time when Rx is between 1 and 2 inches. If a sample with longitudinal channels is tested in this manner, there will be rapid flow in the longitudinal direction as fluid gushes through the channels, but in other flow directions (along paths proceeding radially outward from the periphery of the inlet port) that do not align well with the open channels, the flow will be much slower, resulting in a moving dye boundary that is greatly extended in the direction of the channels but which travels much less in other directions. Such a moving dye boundary will be irregular, possibly asymmetric, and will have long path lengths in some directions but much shorter path lengths in others, yielding xcex1 values over 4. Values of xcex1 as great as 2 may be achieved in machine made papers due to fiber orientation, so a limiting value of 4 has been selected to distinguish the effects of macroscopic nonuniformities from the effects of inherent small scale sheet structures on the measured In-Plane Permeability.
Three-dimensional materials for absorbent articles in which a structure is obtained by folding, pleating, cutting, etc., to generate a macroscopic structure lack the uniform nature of the material of the present invention. While the material of the present invention can be so arranged in various three-dimensional methods, it is important to differentiate the high In-Plane Permeability intrinsic to the materials of the present invention from the possibility of high In-Plane Permeability results obtained from macroscopic structures (those which do not have a representative unit area less than about 15 mm. by 15 mm. using the concept of representative elementary area in the sense known to those skilled in the art of flow through porous media and as explained by Jacob Bear in Chapter 1 of Dynamics of Fluids in Porous Media, Elsevier Publications Company, 1972, or those which do not have a nearly uniform basis weight distribution). For example, a pleated and folded structure may have long, macroscopic channels in the direction of folding which can provide large, open pathways for fluid flow. Such a material could be positioned in such a manner that it would offer little flow resistance in measurements of In-Plane Permeability, for the fluid would be flowing in the open channels, not through the sheet. High In-Plane Permeability results must be obtained in a structure with an xcex1 value less than 4 when measured with dilute aqueous, fugitive dye injection into the dry material, as described previously. An important advantage to having the sheet be uniform with respect to large length scales is that the uniform material provides continuous wicking paths and prevents fluid leakage through large channels. The surface pores and other three-dimensional structures are small enough to still provide capillary transport and good fluid retention, whereas pleated, folded, cut, or other large-scale three-dimensional sheets have channels which are ineffective at capillary transport because of their large diameter and which also can promote leaking.
The radial uniformity of flow in typical materials of the present invention is demonstrated in FIGS. 26 and 27, which are photographs of dye injection experiments, with optical access to the moving dye boundary made possible by a mirror at a 45xc2x0 angle below the Plexiglas support plate of the permeability apparatus. The camera is directed towards the mirror which provides a view of the underside of the clear support plate, where the growth of the dye boundary is visible. In these tests, an aqueous dye solution was prepared from 40 ml of 7% Versatint Purple II dye (Milliken Chemical, Inman, S.C.) added to 1000 ml of deionized water. FIG. 26 shows successive images of the moving dye boundary advancing in a stack of two disks of dry material from the present invention, an uncreped through-air-dried 40 gsm basesheet of spruce BCTMP produced with 30 lbs. of Kymene per ton of fiber. The motion of the fluid is slightly faster in the machine direction, resulting in an elliptical shape aligned with the machine direction of the paper. Application of Equation 6 for FIGS. 26A and 26B results in xcex1 values of 1.70 and 1.76, respectively (edge effects in FIG. 26C have hindered flow in the machine direction, resulting in a lower xcex1 value of about 1.6). FIG. 27 shows a moving dye boundary in a slightly moistened 60 gsm basesheet of spruce BCTMP with 20 lbs. of Kymene added per ton of fiber, made with a T-116-1 throughdrying fabric (Lindsay Wire Division, Appleton Mills, Appleton, Wis.). An cc value of about 1.4 is obtained in this case. These dye injection tests also show that the motion of the dye is through the porous medium and not through large channels in the sheet or through random gaps between the sample and the constraining surfaces.
As will be illustrated in the Examples, the webs of this invention possess very high In-Plane Permeability. More specifically, the In-Plane Permeability can be about 5xc3x9710xe2x88x9211 square meters or greater, more specifically about 8xc3x9710xe2x88x9211 square meters or greater, more specifically about 10xc3x9710xe2x88x9211 square meters or greater, still more specifically from about 5xc3x9710xe2x88x9211 to about 80xc3x9710xe2x88x9211 square meters, and still more specifically from about 8xc3x9710xe2x88x9211 to about 30xc3x9710xe2x88x9211 square meters.
The xe2x80x9cFIFE Testxe2x80x9d is substantially as described in U.S. Pat. No. 5,147,343 issued Sep. 15, 1992 to Kellenberger entitled xe2x80x9cAbsorbent Products Containing Hydrogels With Ability To Swell Against Pressurexe2x80x9d, which is herein incorporated by reference. For purposes herein, the FIFE Test is carried out as described except for the following differences: the raised platform on the lower plate has been removed, so the lower surface is entirely flat; the sample area is 8 inches square and the blotter paper sheets are also cut to this size; multiple sheets were used to obtain a stack with a basis weight of about 240 gsm (roughly 10 grams total mass); samples were tested with a thin layer of poly film beneath them to enable them to be picked up more easily for flowback measurement; and each liquid insult was 40 milliliters. Since actual sample masses will vary slightly from the target of 10 grams, insult times are normalized to a mass of 10.0 grams by multiplying each observed intake time by a factor of (conditioned dry sample mass/10 grams). The combined time of the first, second and third insults is the FIFE Test value of the sample.
The webs of this invention can have FIFE Test values of about 125 seconds or less, more specifically about 75 seconds or less, still more specifically about 25 seconds or less, and still more specifically from about 25 to about 100 seconds.
The xe2x80x9cDry Wipe Residuexe2x80x9d test provides a means of quantifying the ability of a web to wipe a surface dry. This property is of particular interest for products such as wipes, paper towels, cleaning articles and absorbent articles. To carry out this test, a piece of material approximating kitchen towel dimensions is attached to an 8 inchesxc3x978 inchesxc3x97xc2xd inch aluminum plate with adhesive tape. The material is wrapped over the top of the plate and taped there. There is a xc2xd inch diameter hole in the center of the plate. The plate with the material wrapped over it is illustrated in FIG. 17. It is placed on a 10 inchesxc3x9712 inchesxc3x97xe2x85x9 inch clear glass plate. A 3 cubic centimeter insult of 0.5% MBNS dye solution (Keystone Aniline, Chicago, Ill., available at 10.5% solids) is imparted into the hole and 10 seconds is allowed for absorption. The plate is then picked up vertically and the pattern on the glass allowed to dry (about 20 minutes). The glass plate is then placed dye-side down on a sheet of pink paper (Neenah Bond 02651, Neenah Paper Company, Neenah, Wis.) to provide optical contrast for imaging the blue dye against the paper when viewed through the opposite side of the glass plate. The residues are imaged with a Quantimet 900 Image Analysis system (Leica, Inc., Deerfield, Ill.) using the optical set-up and conditions shown by the following routine, xe2x80x9cWIN1xe2x80x9d.
The results of the Dry Wipe Residue testing provide a Total Area coverage for the residue, a percent area coverage of the residue and a Mass Factor (area * darkness/1000) which represents the mass of material in the residue. The xe2x80x9cTotal Areaxe2x80x9d of the residues pattern is simply the sum of all xe2x80x9cblackxe2x80x9d pixels in the image of the residues pattern, regardless of how deeply black they might be. In practice, the detection level is set to accept all pixels from 0 to about 55 (adjust as needed) on a 6-bit gray scale running from 0 to 64. The xe2x80x9cMass Factorxe2x80x9d is a parameter that weights the area of the image of the residues by the mean gray level underlying all pixels and dividing by 1000 to generate more manageable numbers. This has been shown to give a value approximately proportionate to the mass (milligrams) remaining on the glass plate, provided that optical saturation does not occur (i.e., there are not very dark residues regions present). The xe2x80x9cpercent coveragexe2x80x9d is simply 100 times the ratio of the area of the xe2x80x9cblackxe2x80x9d pixels within the boundary of the image xe2x80x9cTotal Areaxe2x80x9d to the entire area enclosed in a cover region drawn around the residues pattern using the mouse editor.
Webs of this invention can have Dry Wipe Residue Total Area coverage values of about 2000 square mm. or less, more specifically about 1500 square mm. or less, and still more specifically from about 500 to about 1000 square mm. The Dry Wipe Residue Mass Factor can be about 30 or less, more specifically about 20 or less, and still more specifically from about 5 to about 20 or 30.
A test similar to the Dry Wipe Residue test just described is the xe2x80x9cWet Wipe Residuexe2x80x9d test, which is conducted with an initially saturated wet sheet rather than starting with a dry sheet. Specifically, a 3 inchesxc3x973 inches piece of the test material is truncated at ⅓ edge distances to form an octagon as illustrated in FIG. 18. The octagonal test material is placed in a 100 millimeter crystallization dish and saturated to 350 weight percent with a 0.5% MBNS solution as described above. The area of the octagon is about 7 square inches. The dwell time for saturation is 3 minutes. The saturated material is picked up with a tweezers and placed on a 10 inchesxc3x9712 inchesxc3x97xe2x85x9 inch clear glass plate. An 8 inchesxc3x978 inchesxc3x971 inch piece of aluminum is placed on top of the material and allowed to dwell for 30 seconds. The plate is then picked up vertically and the test material removed from the glass plate with tweezers. The residues are allowed to dry (about 5 minutes). (See FIG. 19.) The plate is placed dye-side down on a sheet of pink paper as described above and the residues are imaged with a Quantimet 900 Image Analysis system using the same optical set-up and imaging conditions shown by the xe2x80x9cWIN1xe2x80x9d routine identified above.
As with the Dry Wipe Residues test, the same xe2x80x9cWIN1xe2x80x9d routine yields values for Total Area and percent area coverage by the residues and a Mass Factor for the residues. Webs of this invention can have a Wet Wipe Residue Total Area coverage of about 1500 square mm. or less, more specifically about 1000 square mm. or less. and still more specifically from about 400 to about 800 square mm. The Mass Factor for the Wet Wipe Residue test can be about 5 or less, more specifically from about 2 to about 5.
Some of the webs of this invention may also be characterized in part by the xe2x80x9cMean Volume-Weighted Pore Lengthxe2x80x9d, expressed in microns. This structural parameter is related to the wicking ability of the material when wetted. The Mean Volume-Weighted Pore Length is determined by placing a 6 inchesxc3x976 inches piece of the material to be tested on a plastic sheet (e.g. xe2x80x9cGlad Wrapxe2x80x9d or similar material) on a horizontal flat surface. The sample is then flooded with distilled water. The material is allowed to dry over night at less than 40% relative humidity. Subsections of the dried material are cut off and cross-sectioned under liquid nitrogen for back-scattered electron photomicroscopy as described in U.S. Pat. No. 5,492,598 issued Feb. 20, 1996 to Hermans et al. entitled xe2x80x9cMethod for Increasing the Internal Bulk of Throughdried Tissuexe2x80x9d, which is herein incorporated by reference. However, for purposes of measuring Mean Volume-Weighted Pore Length, only 7 photos are taken at a constant 50xc3x97 magnification for all samples. The photos are not assembled into a photomontage, but placed individually under plate glass on a Kreonite Macroviewer (J. Kelly, Inc., Darien, Ill.) and viewed with a 50 mm. EL-Nikkor lens (Nikon, Inc., OEM Sales Group, Melville, N.Y.). The image is oriented horizontally across the photo as illustrated in FIG. 23 and analyzed by the routine xe2x80x9cTSAI3xe2x80x9d which follows below. The cross-section boundaries are selected by ACCEPT and REJECT operations using xe2x80x9cmousexe2x80x9d EDIT on the Quantimet 970 Image Analysis System (Leica, Inc., Deerfield, Ill.). The parameters are described by equations in xe2x80x9cTSAI3xe2x80x9d.
Some of the webs of this invention can have a Mean Volume-Weighted Pore Length of about 550 microns or greater, more specifically about 700 microns or greater, and still more specifically from about 600 to about 1000 microns.
Additionally, the webs of this invention have a substantially uniform thickness, as evidenced by a relatively low thickness percent coefficient of variation (% COV) referred to herein as the Thickness Variation Index. The Thickness Variation Index can be about 25 percent or less, more specifically from about 5 to about 15 percent. To determine the thickness percent coefficient of variation, photomicrograph montages of tissue cross-sections are prepared by the scanning electron-microscopy method described in U.S. Pat. No. 5,492,598 incorporated by reference above (on tissues that were not previously wetted). For this method, however, montages need not be assembled (since autostage control is unnecessary) and the ideal magnification of 50xc3x97 can be held constant across all photos. Individual photos were viewed in horizontal orientation (FIG. 28A) with a 50 mm. EL-Nikkor lens that provides a 2xc2xc inches field of view with the Chalnicon scanner attached to a Quantimet 970 Image Analysis System. Illumination is provided by a Kreonite Macroviewer using 4 photo-flood lamps. Using the routine xe2x80x9cTSAI2xe2x80x9d set forth below, the image of the tissue cross-section is filled as a solid detection region (FIG. 28B) by various binary operations and then xe2x80x9cLINExe2x80x9d slices are taken at local maxima and minima to represent thickness samples (FIG. 28C). These are assembled into a histogram from which MEAN and standard deviation values are extracted for % COV=100("sgr"/xcexc).